A California startup wants to sell sunlight on demand, beaming it to Earth from orbiting mirrors. Astronomers say the plan could ruin the night sky.

Reflect Orbital plans to launch a test satellite called Earendil-1 later this year with an 18-by-18-metre mirror. The US Federal Communications Commission approved the demonstration in July. The company eventually wants up to 50,000 larger mirrors in orbit.

The idea is to reflect sunlight onto areas up to 5 kilometres wide for uses such as extending solar-farm power generation, lighting disaster zones and search-and-rescue work. The company says the service would be highly localised and temporary.

A new study found that a single satellite would appear about 40 times brighter than the full moon inside its target area, and as bright as the full moon up to 14 kilometres away. ‘This is incompatible with astronomy,’ said astronomer Samantha Lawler.

Who gets to decide how darkness and low-Earth orbit are used is still an open question, and regulators have yet to settle it.